I continued to monitor the gravity, pH and temperature after the last addition. All measurement were done in the evening so that the intervals between measurements were as close to 24 hour as possible
Thursday the 30th of October
Sg: 1.040
pH: 3.69
Temp: 23.6 C
Friday the 31th of October
Sg: 1.022
pH: 3.68
Temp: N.A.
Saturday the 1st of November
Sg: 1.010
pH: 3.70
Temp: 22.7 C
So how does this SNA batch compare with the cyser. When plotting the gravity readings you get
Basically I had estimated that that a drop of 0.100 would correspond to 14 %. Hence if the yeast max out at 15%, it should not change gravity by more than 0.107 and should in this case have stopped at 1.013. Now it is 1.010 and it is still going strong even though the rate has been reduced a bit
The simple formula for calculating Alcohol by volume is:
ABV= (OG-FG)/0.00753 (ref wikipedia)
This basically says that the gravity changes by 0.00753, then 1% of alcohol is produced.
For high gravity there is a somewhat more complicated formula.
ABV =76.08*(OG-FG)/(1.775-OG)*FG/0.794
The idea is that more ethanol is produced at the end of the fermentation since the replication rate reduces. This means that more alcohol should be produces per gravity unit.
The high gravity formula (alternative formula) as it is returns ridiculously high ABV values – far higher than the yeast max, so I just adjusted the simple formula by using a factor of 0.0071. So 15%max x 0.0071 = 0.107.
Right now I am back to the original simple formula. Then, for an ABV of 15% the new low is 1.007 instead of 1.013.